An emulsion polymerized PTFE is obtained by polymerizing the tetrafluoroethylene (hereinafter referred to as TFE) monomer, in the presence of pure water, a polymerization initiator, a fluorine-containing anionic surfactant (hereinafter referred to as APFO) such as ammonium perfluorooctanoate and a paraffin wax stabilizer, and it is obtained as an aqueous PTFE polymer emulsion containing PTFE fine particles (Handbook of Fluorine resins, p. 28, edited by Takaomi Satokawa, published by Nikkankogyo Shimbun).
The aqueous PTFE polymer emulsion after the polymerization is used as a crude aqueous PTFE dispersion after blended with a nonionic surfactant, or as a blended aqueous dispersion of PTFE after blended with a nonionic surfactant, a filler or other known components as the case requires. However, it has not been industrially common to remove APFO from an aqueous dispersion of PTFE. Because APFO is hardly naturally degradable, it is preferred to minimize the APFO content of products.
As to means of lowering the APFO content, WO00/35971 (JP-A-2002-532583) proposes a method of preparing an aqueous dispersion of PTFE with a low APFO concentration by passing a crude aqueous dispersion of PTFE through a column packed with an anion exchange resin (hereinafter referred to as AER) and allowing the AER to absorb APFO. In this method, it is important to increase the APFO absorption per AER to lower costs because the AER is generally expensive. However, The APFO absorption disclosed in WO00/35971 is relatively low at the level of less than 16% of the theoretical capacity, and there is no disclosure about how to increase the absorption. Further, because strong basic AERs are used in the Examples, there is a problem that the efficiency of the removal of APFO tends to decrease during continuous passage of a crude aqueous dispersion of PTFE due to agglomeration of the PTFE fine particles in the column.
Besides, though it is not disclosed how to feed the crude aqueous dispersion of PTFE to the column, when an ordinary mechanical pump which generates shearing force is used to feed the dispersion, there is a problem that agglomerates of PTFE fine particles accumulate in the AER column to hinder removal of APFO. Though it is preferred to elute the APFO absorbed by the AER with an eluent such as an alkaline solution for reuse, there is a problem that the strong basic AER used in the Examples absorbs APFO too strong to elute APFO.